GEOG 100--Weathering and Mass Wasting (F'13)


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GEOG 100--Weathering and Mass Wasting (F'13)

  1. 1. Weathering and Mass Wasting Chapter 10
  2. 2. External vs. Internal Processes (the dynamic equilibrium model) (the dynamic equilibrium model) 2
  3. 3. Dynamic Equilibrium • Equilibrium stability (fluctuating around some average) [Geomorphic threshold is reached] • Destabilizing event • Adjustment • New condition of equilibrium stability 3
  4. 4. The Grand Canyon 4
  5. 5. Denudation—Large-scale removal of material that lowers the overall profile of the topography 5
  6. 6. Denudation Processes • Weathering—The combined action of all atmospheric and biologic processes that cause rock to disintegrate physically and decompose chemically because of exposure near Earth’s surface (from bedrock to regolith) • Mass wasting—spontaneous downslope movement of soil and eroded rock fragments under the influence of gravity, but without the action of moving air, water or ice • Erosion—extensive removal of rock material, generally transported long distances 6
  7. 7. Weathering of Bedrock ► Wherever bedrock is exposed to the natural elements, it weathers ► Any crack, joint, or cavity in the rock will allow weathering agents to penetrate and break it apart 7
  8. 8. Jointing in Bryce Canyon, UT
  9. 9. As rocks weather, surface area increases, offering more surfaces to be weathered…
  10. 10. …producing this result.
  11. 11. Mechanical Weathering • Physical disintegration of rock as a result of natural phenomena, without a change in its chemical composition – Pounding, pushing, cracking, breaking, wedging apart 13
  12. 12. Mechanical Weathering Processes • • • • • Frost wedging Salt wedging Unloading/pressure-release jointing Thermal expansion and contraction Biologic weathering
  13. 13. Frost Wedging • Repeated growth and melting of ice crystals in pore spaces of rock fractures or joints • Expanding ice exerts pressure, breaking rocks apart • Most effective where there is repeated freeze and thaw (as in arctic or tundra environments)
  14. 14. Frost Wedging
  15. 15. Salt Wedging • Similar to frost wedging • Growth of salt crystals breaks rocks apart • Most effective in coastal environments and semi-arid environments Honeycomb (tafoni), Salt Point, Sonoma Coast
  16. 16. Unloading or Pressure-release jointing • Rock brought near the surface as the rocks above (or even glaciers) erode away relieves confining pressure and allows the rock to expand slightly, forming cracks – Sheeting—The breaking away of layers of rock in sheets, caused by expansion, usually from unloading processes • Exfoliation dome—Sheeting on a massive scale, over the face of a large segment of rock (Half Dome in Yosemite, Sierra Nevada Mtns.) 18
  17. 17. Thermal Expansion and Contraction • When rock is heated, it expands slightly, and when cooled, it contracts • Rapid expansion and contraction of the surface of the rock causes cracks to form and propagate • Most effective in regions with large differences in temperature between daily highs and nightly lows
  18. 18. Thermal Expansion and Contraction
  19. 19. Biologic Weathering • Growth of plant roots, burrowing animals • Pressure is exerted by the growth of tiny rootlets in joint fractures, which causes the loosening of small rock particles and mineral grains • Burrowing animals such as squirrels and oysters may also erode rocks
  20. 20. Chemical Weathering • Decomposition of rock through chemical alteration of its minerals • Exposed to water or other solutions, minerals in rocks undergo a chemical change, weakening internal structures – Air, soil water solutions, and groundwater solutions contain dissolved oxygen, carbon dioxide, or other reactive elements – Water is the greatest agent of chemical weathering
  21. 21. Chemical Weathering: The Influence of Temperature and Precipitation Chemical weathering is most effective in warm, moist climates
  22. 22. Oxidation • Oxygen dissolved in soil water or ground water can bond with the chemical elements of the minerals to form new minerals • Causes expansion and exerts pressure that breaks the rocks apart • Example: iron (Fe) turning to rust (Fe2O3) in the presence of oxygen and water
  23. 23. Hydrolysis and Hydration • Hydrolysis--minerals reacting with water split into other compounds (may also split the water molecules) – granite: feldspar turns to clays + quartz sand – contributes to spheroidal weathering
  24. 24. Hydrolysis and Hydration • Hydration--The whole water molecule forms chemical bonds to become part of the chemical composition of the rock, causing expansion and grain-by-grain destruction of rocks Formation of gypsum from anhydrous calcium sulfate (the mineral anhydrite) which has absorbed water into its chemical structure
  25. 25. Carbonic Acid Carbon dioxide dissolved in water creates a weak acid called carbonic acid which can dissolve some minerals, especially calcium carbonate Limestone and marble are most susceptible to this type of weathering
  26. 26. Acid Precipitation • Urban pollution from sulfur and nitrogen oxide gases mixes with atmospheric water, forming acid precipitation • Dissolves limestone and marble (often used for public statues and tombstones) and other types of building stones; destroys vegetation, affects human health
  27. 27. Organic Acids • Decaying vegetation mixes w/ water to form soil water w/ complex organic acids that can react to dissolve or chemically alter minerals
  28. 28. Mass Wasting • Material is moved a short distance down a slope under the influence of gravity • Angle of repose—the steepest angle that loose fragments can lie without movement if undisturbed
  29. 29. 31
  30. 30. Mass Wasting • The type of mass wasting event that occurs will depend upon speed and the degree of saturation
  31. 31. Mass Wasting (another view)
  32. 32. Types of Mass Wasting • Rock fall • Rock slide and Topple • Slump • Solifluction • Creep • Debris flow • Earth flow • Mudflow • Induced mass wasting
  33. 33. Rock Fall Talus slopes—Regolith which has fallen down steep slopes, funneled into “blankets” of rock called talus cones Fresh slopes are very unstable
  34. 34. Rock Fall
  35. 35. Rock Slide
  36. 36. Mudflow and Debris Flow • Mudflow—Rainwater mixed with soil flowing very quickly downslope as a river of mud – Usually in canyons of mountainous regions – Can carry large objects, destroying property and taking lives – Flows until mud thickens, slows, and eventually stops • Debris flow—More rock fragment than mudflow, but similar in other characteristics
  37. 37. Earthflow • Water-saturated soil or rock material • Moves a limited distance down slope as one large mass • Generally slower in motion (over the course of hours) • Common form of earth movement causing road closures and property destruction during heavy rains
  38. 38. Near La Conchita Slide, along Hwy. 101 in Ventura County Hwy. 101 in Ventura County
  39. 39. La Conchita Slide (Earthflow) Hwy. 101, Ventura County
  40. 40. La Conchita Slide (Earthflow) Hwy. 101, Ventura County
  41. 41. Slump—Slow, concave sliding
  42. 42. Slump
  43. 43. Slump
  44. 44. Solifluction • Continuous freeze and thaw cycles slowly move weathered particles downslope • Over time, the entire slope moves downhill
  45. 45. Solifluction
  46. 46. Solifluction
  47. 47. Soil Creep
  48. 48. Induced Mass Wasting Mass wasting caused by human activity • Moving weathered rock material downslope during construction on steep hillsides – Carried away as debris flows or mudflows during heavy rains • Removal of material supporting the base of a slope • The wetting of weathered rock material and soil from pipe breakage, lawn watering, etc. causing slippage • Debris removal by heavy rains after fire may also remove stabilizing vegetation
  49. 49. Induced Mass Wasting: Construction of the Panama Canal